1. Field of the Invention
The present invention relates to an optical data-processing apparatus of the type which is provided with an objective lens facing an optical data storage medium for making a light spot on the storage medium. In particular, the present invention relates to a technique applicable for such a data-processing apparatus whereby spherical aberration due to the thickness error of the substrate of the storage medium is properly detected. In this specification, an xe2x80x9coptical data storage diskxe2x80x9d may refer to any type of data storage medium with which desired information is written or read out optically. For instance, the optical storage medium may be a read-only disk (such as CD-ROMs), magneto-optical disk or phase change optical disk.
2. Description of the Related Art
For detection of a focus error in an optical data-processing apparatus, Foucault method is often employed. This method can be implemented in a conventional optical data-processing apparatus as shown in FIG. 11 of the accompanying drawings.
Specifically, in the conventional apparatus, a laser beam emitted from a laser diode 90 passes through a collimating lens 91, a first beam splitter 92a and an objective lens 93, to strike upon an optical data storage disk D. The laser beam, after reflected on the disk D, passes through the objective lens 93 again and enters the first beam splitter 92a. This time, the laser beam is reflected in the beam splitter 92a, to be directed toward a second and a third beam splitters 92b, 92c. In the splitters 92b and 92c, as shown in FIG. 11, the laser beam is partly reflected (upward in the figure) and partly allowed to pass through. The reflected light in the second beam splitter 92b is led to a magneto-optical signal detector, while the reflected light in the third beam splitter 92c is led to a tracking error detector.
The laser beam having passed through the splitters 92b and 92c is led to a compound prism 94 and to a focus error detector which incorporates a light detecting device 95. Then, a focus error signal is generated by the Foucault method in the manner described below.
Referring to FIG. 12, as passing through the compound prism 94, the laser beam splits into an upper ray and a lower ray both of which have a semicircular cross section. These two rays are detected by the light detecting device 95. In the illustrated situation, when a focus error occurs, the two semicircular light spots on the detecting device 95 shift in position. The detecting device 95 has a light-receiving surface quartered into firstxcx9cfourth sections axcx9cd by two division lines Lx and Ly perpendicular to each other. Each of the four sections axcx9cd receives light, to generate a detection signal corresponding to the amount of the received light. The signals outputted from the detecting device 95 are supplied to a focus error signal generator (FESG) 96 to produce a focus error signal (FES). The focus error signal has a level LFES equal to {(Laxe2x88x92Lb)+(Lcxe2x88x92Ld)}, where Laxcx9cLd are the levels of the detection signals outputted from the sections axcx9cd, respectively.
The Foucault method will now be described with reference to FIGS. 13Axcx9c13B, 14Axcx9c14B and 15Axcx9c15B.
When the focusing of the objective lens 93 is proper (FIG. 14A), each of the two beam spots on the detecting device 95 has an oval form that is symmetrical with respect to the horizontal division line Lx. In this case, the LFES becomes zero. However, when the objective lens 93 is too close to the disk D (FIG. 13A), the two beam spots will take a form and a position as shown in FIG. 13B. In this case, the LFES becomes greater than zero. On the other hand, when the lens 93 is too distant from the disk D (FIG. 15A), the two beam spots will take a form and a position as shown in FIG. 15B. In this instance, the LFES becomes smaller than zero.
As seen from the above, the focus error signal can be used for detection of the defocusing of the objective lens 93. More precisely, it is possible to detect the extent and direction of the defocusing of the lens 93 based on the focus error signal (FES). Thus, the focus control for the lens 93 can be performed based on the FES, whereby the lens 93 is moved toward or away from the disk D (i.e., in the focus direction) for focus adjustment.
A typical optical disk may include a transparent substrate and a recording layer formed on this substrate. In using such an optical disk, the laser beam is first led through the transparent substrate and then shone on the recording layer. Unfavorably, the substrate of an optical disk may lack uniformity in thickness (i.e., the substrate has a thickness error), which causes spherical aberration. Spherical aberration makes it difficult to bring the objective lens to the right focus position in performing the focus control. Accordingly, it is impossible to make a sufficiently small light spot on the storage disk, and therefore the required data-recording or data-reading cannot be performed. Recently, a high NA objective lens (NA stands for xe2x80x9cnumerical aperturexe2x80x9d) is preferred for increasing the data storage density of the storage disk. However, since the spherical aberration is proportional to the fourth power of the NA, the apparatus incorporating a high NA objective lens may suffer unacceptably large spherical aberration. In the past, no easy but accurate technique has been proposed for detecting spherical aberration caused by the substrate thickness error.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an optical data-processing apparatus whereby the occurrence of spherical aberration can be detected easily and accurately.
According to a first aspect of the present invention, there is provided an optical data-processing apparatus that includes: an objective lens for convergence of light beams emitted from a light source to make a beam spot on a recording layer of an optical data storage medium; a first light splitter for splitting reflected light from the storage medium into two semicircular rays; a second light splitter for splitting the two semicircular rays into non-biased light and biased light which has a different optical path length than the non-biased light; an optical detector that receives the non-biased light and the biased light, thereby producing a first signal corresponding to the received non-biased light and a second signal corresponding to the received biased light; a first signal processing unit for generating a focus error signal based on the first signal; and a second signal processing unit for generating a spherical aberration signal based on the second signal.
In the above data-processing apparatus, the focus error signal, which is obtained on the basis of the above-mentioned non-biased light, may be produced by the Foucault method as in the prior art discussed above. The spherical aberration signal, on the other hand, is obtained on the basis of the above-mentioned biased light. Since the biased light has an optical path length different from the counterpart of the non-biased light, the profile of the biased light will change when spherical aberration occurs. Based on this profile change, the spherical aberration signal is obtained. According to the present invention, both a focus error signal and a spherical aberration signal are obtained simultaneously. Thus, while the focus control is being performed, spherical aberration control can also be performed. As a result, an appropriately small beam spot can be formed on the recording layer of the storage medium, which is advantageous to performing proper data writing or data reading with respect to the storage medium.
Preferably, the biased light split by the second light splitter may include plus 1-order diffracted light and minus 1-order diffracted light. In this case, the second signal processing unit may generate the spherical aberration signal based on both the plus 1-order diffracted light and the minus 1-order diffracted light.
With the above arrangement, it is possible to produce a spherical aberration signal which more accurately reflects the properties of the actual spherical aberration.
According to a second aspect of the present invention, there is provided an optical data-processing apparatus which includes: an objective lens unit for convergence of light beams emitted from a light source to make a beam spot on a recording layer of an optical data storage medium; a focus error detector that produces a focus error signal based on reflected light from the storage medium; and a spherical aberration detector into which the reflected light is introduced, the spherical aberration detector being provided separately from the focus error detector. The spherical aberration detector includes: a beam splitter for splitting the reflected light into two beams; a converging lens for convergence of the two beams; a first and a second optical detecting devices for receiving the two beams, the first detecting device and the second detecting device being disposed at different distances from the above converging lens; and a signal processing unit for generating a spherical aberration signal based on intensity distribution of the beams received by the detecting devices.
With the above arrangement, when spherical aberration occurs, the intensity distribution of the light received by the first detecting device is different from the intensity distribution of the light received by the second detecting device. Based on this difference, it is possible to detect the occurrence of spherical aberration. As in the apparatus of the first aspect, focus control and spherical aberration control can both be simultaneously performed.
Preferably, the objective lens unit may be movable in a focus direction and supports a first and a second lenses aligned in the focus direction, the first lens being movable in the focus direction relative to the second lens.
Further, the first lens and the second lens may be simultaneously moved for focus control in the focus direction based on the focus error signal. Also, the first lens may be moved relative to the second lens based on the spherical aberration signal for reduction of spherical aberration.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.